This invention relates to rigid polyurethanes having low water absorption prepared by the resin transfer molding method using polyols based on long-chain fatty acid or fatty alcohol groups.
Resin transfer molding is a form of liquid composite molding in which the reaction conditions typically involve longer reaction times and lower pressures than the commonly used reaction injection molding ("RIM") technique. Consequently, resin transfer molding is particularly suited to the production of fiber-reinforced plastic articles, for which wetting and impregnation of the reinforcement material is often difficult, and the production of very large articles, for which the high pressure molds used in the RIM technique are impractical or unavailable. See, for example, E. B. Stark and W. V. Breitigam, "Resin Transfer Molding Materials," and C. F. Johnson, "Resin Transfer Molding," in Engineered Materials Handbook, Vol. 1 ("Composites") (Metals Park, Ohio: ASM International, 1987), pages 168-171 and 565-568, respectively; G. R. Smoluk, Modern Plastics, 66, 57-65 (January, 1989); J. K. Rogers, Plastics Technology, 35, 50-58 (1989); and R. V. Wilder, Modern Plastics, 66, 48-50 (July, 1989). Although the resin transfer molding technique has most commonly been used for polymers other than polyurethanes, the technique can also be used with reactive urethane-based systems. For example, U.S. Pat. Nos. 4,695,509, 4,919,876, and 5,009,821 disclose the preparation of polyurethane composites and U.S. Pat. Nos. 4,757,123 and 4,800,058 disclose the preparation of modified rigid polyisocyanurate polymer compositions by resin transfer molding, and U.S. Pat. No. 5,277,862 discloses the preparation of certain polyurethane elastomers.
Resin transfer molding, however, involves relatively long gel times and is thus often characterized by poor mold release and formation of bubbles. Therefore, an object of the present invention was to find a process for preparing rigid polyurethanes having improved physical properties. It has now been found that the use of certain isocyanate-reactive fatty acid polyester polyols in the resin transfer molding process produces high quality rigid polyurethanes having reduced water absorption.